1. Field of the Invention
The present invention relates to a toner for electrophotography which is used in printers or coelectrophotography machines utilizing heat-roll fixing. Moreover, the present invention relates to a polyester resin used in the toner for electrophotography, and to a process for preparing the same.
2. Description of the Related Art
Recently, various improvements or modifications on coelectrophotography machines or printers utilizing electrophotography have been desired as these machines have become more popular. For example, it has been desired to reduce energy consumption in order for these machines to be practical for home use. It is also desirable to make the machines run at high speeds in order to promote the spread of the machines into so called gray areas between printers and copiers. It is also necessary to simpler the structure of the machine in order to reduce the cost of the machines. Moreover, it has become popular to use the machines having functions of printing on both sides of the printed sheet and delivering the printed sheet automatically. Of the above-described requirements, particularly desired is a toner for electrophotography (hereafter, referred to simply as "toner") having a lower fixing temperature, a good anti-offset quality and Kin strength so that spots are not generated in a printed sheet in printing both sides of the printed sheet.
In order to achieve the requirements in the past, the following were suggested, such as improving a molecular weight and a molecular weight distribution of a binder resin.
Specifically in order to give a good low temperature fixing property to the toner, it is preferable to use the binder resin having a lower molecular weight. By using a resin having a lower molecular weight as a binder resin such as styrene-acrylic resin, the fixing property at low temperatures can be improved; however, the toner comprising styrene-acrylic resin is fragile, and the toner is easily crushed by a stress in a developing machine. As a result, a distribution of a particle size of a toner is changed, or a crushed toner easily coagulated to a carrier particle or a developing sleeve; and when the toner is used for a copying over a long period, the image quality unavoidably deteriorated by changing a charging property. Moreover, a spot is generated in printing on both sides of the printed sheet and delivering the printed sheet automatically.
On the other hand, when the molecular weight of a polyester resin was to be lower, the melting point thereof became lower, simultaneously the viscosity thereof became lower, therefore, an offset phenomena to a fixing roller was generated. In order to prevent generating the offset phenomena, it was suggested to introduce a crossing structure to polyester resin for widening the molecular distribution of polyester resin. The molecular weight distribution can be widen by introducing a crosslinking structure, therefore, the generation of the offset phenomena can be prevented. However, the total molecular weight range of polyester resin also became wider, therefore, the fixing properties of the toner comprising such polyester resin became worse. Consequently, in order to give sufficient fixing properties to the toner, it was necessary to lower a glass transition temperature (Tg) of the resin. Therefore, it is difficult to avoid deteriorating the storage properties of the toner comprising the resin. In this way, it is difficult to provide a toner having anti-offset properties, satisfactory storage properties, and low temperature fixing properties.